An ultrasound imaging system is a device that is widely used in a medical field. In the ultrasound imaging system, ultrasound signals are transmitted to tissues in a target object like a human body, and the ultrasound signals reflected from the tissues are transformed to receive signals. An ultrasound image is formed with these receive signals.
The ultrasound image is mainly expressed with a brightness-mode (B-mode) based on reflection coefficients which vary with impedance differences between the tissues in the target object. However, it is hard to observe a lesion, such as a tumor or a carcinoma, with the B-mode image, because the reflection coefficients of the tumor or the carcinoma are not so much different from those of other tissues adjacent thereto.
Meanwhile, eleastrography for forming ultrasound elasticity images, i.e., strain images, utilizes mechanical characteristics of the tissues, which are difficult to be observed in the B-mode image, and thus the eleastrography can be a great assistance for the diagnosis of the lesion.
The mechanical characteristics of the tissues can be obtained by comparing a first receive signal and a second receive signal which are respectively obtained without and with applying stress, i.e., a force per a unit area, to the target object. The first and the second receive signals have a RF form.
Due to the stress, each tissue has different displacement, which reflects the mechanical characteristics, for example, hardness of the tissue. The displacement can be obtained in consideration of a phase difference or a delay between the first and the second receive signals. In the elastography, the displacement of each tissue can be obtained by computing a cross correlation or an autocorrelation of the first and the second receive signals. In the cross correlation, the first and the second receive signals in the RF form are computed. However, in the autocorrelation, the first and the second receive signals should be converted into I/Q baseband signals.
The autocorrelation has an advantage of enhancing speed of the computation because the data amount of the I/Q baseband signals is less than that of the RF signals. However, the autocorrelation expresses the displacement in terms of a phase value, and thus an additional step is required to convert the phase value into a time value.
A center frequency of ultrasound transmission signals is used to convert the phase value into the time value. The center frequency varies with the depth of the tissue in the target object, and consequently an error will arise if a fixed value is used as the center frequency during the conversion of the values.
In case that the phase value is computed by using the autocorrelation, aliasing is generated when the phase difference of first and the second receive signal is greater than ½ wavelength of the ultrasound transmission signals. Therefore, an additional process should be introduced to compensate for the aliasing.
As the depth of the tissue becomes deeper, the first and the second receive signals are increasingly different in phase and shape, and thus error also increases due to the decorrelation of the first and the second receive signals.